Monitoring of patient vital signs is used in various medical environments, such as hospital wards, operating rooms and Intensive Care Units (ICUs). Vital-signs monitors, also referred to as Patient Monitors (PMs), typically monitor physiological parameters such as temperature, heart rate and peripheral oxygen saturation (SpO2) in the blood, among others.
Many vital-signs monitors include a standard plug for connecting to a thermistor-based temperature probe. Probes of this type were originally developed and standardized by YSI Inc. and include the YSI 400 and YSI 700 types. Such probes include a thermistor sensor, with a calibrated temperature response, and a cable with a standard connector for plugging into the monitor. The vital-signs monitor simply measures the resistance value across the output connector of the cable. The monitor calculates and displays the temperature according to the measured resistance and the known calibration curve.
Patient temperature measurement can be performed using various kinds of temperature sensors. Some types of temperature sensors are non-invasive, and typically measure the body-surface temperature. Non-invasive temperature sensors are described, for example, in U.S. Pat. No. 7,625,117 and in U.S. Patent Application Publication 2009/0299682, whose disclosures are incorporated herein by reference.
U.S. Pat. No. 7,641,390, whose disclosure is incorporated herein by reference, describes a method for digitally controlling the resistive output of a temperature probe. The method uses a temperature sensor, a processor and means under the control of the processor for modifying the resistive output, such as a digital potentiometer. In one embodiment, the processor reads the temperature sensor and adjusts the potentiometer based on a correlative or predictive technique so as to provide a modified output that matches that of a standard resistive temperature probe and is compatible for display on a multi-parameter monitor.
U.S. Pat. No. 7,484,887, whose disclosure is incorporated herein by reference, describes an interface for a monitor and a temperature probe including a temperature sensor. The interface includes a logic circuit for determining a modified resistive output for the temperature sensor and a means for providing the modified resistive output. The means for providing the modified resistive output includes a Field-Effect Transistor (FET), which is coupled to the logic circuit via a first terminal and via a feedback arrangement, providing a FET resistance corresponding to the modified resistive output.
U.S. Patent Application Publication 2012/0065540, whose disclosure is incorporated herein by reference, describes a temperature sensor with calibrated analog resistive output. In some embodiments, a thermometric apparatus includes at least one body-surface sensor, which is configured to be placed at a location on a body surface of a patient and generates a sensor output that varies according to a body-surface temperature at the location. Analog conversion circuitry is coupled between the at least one body-surface sensor and a connector for coupling to a patient monitor. The circuitry is configured to convert the sensor output into an output resistance across the connector that is indicative of a corrected temperature of the patient.